Hydraulically actuated, electronically controlled, unit fuel injection systems (HEUI) use an actuating fluid (the actuating fluid preferably being engine lubricating oil, but other fluids are acceptable) rail to provide actuation actuating fluid to each injector for generating high pressure fuel for the injection process. The actuating fluid rail typically has its actuating fluid supply provided by a high-pressure actuating fluid pump driven by the engine drive shaft. The pressure in the actuating fluid rail is typically controlled by an Injection Pressure Regulator (IPR) valve, which determines the actuating fluid pressure in the rail depending on engine operating conditions.
Each injector has an actuating fluid control valve that is electronically controlled to control the timing and amount of the actuating fluid flowing into the injector. The actuating fluid control valve initiates and terminates the injection process.
V-form engines typically have a separate actuating fluid rail servicing each of the two banks of cylinders respectively. At the actuating fluid flow inlet of each rail, there may be a check valve in place to isolate the fluid communications between the separate rails servicing the two banks. For a V8 configuration, there are two rails with four injectors attached to each rail. For a V6 configuration, there are also two rails, but with three injectors attached to each rail. For an inline (typically I6) configuration, there is only one rail with six injectors attached to it and there is no check valve at the actuating fluid flow inlet as no rail isolation is needed for a single rail configuration.
The prior art actuating fluid rail is a single elongate straight tube that preferably has a cylindrical shape and a generally cylindrical fluid passageway defined therein and having a volume of about 15 cu. in. The actuating fluid is able to flow freely in the fluid passageway with the least amount of flow restrictions between the locations where injectors are connected to the rail. For the V8 and V6 configuration, the two actuating fluid rails are both connected through actuating fluid flow passages to the high-pressure actuating fluid pump, but separated by the aforementioned check valves at the inlet of the respective rails. These check valves provide isolation between the two actuating fluid rails for limiting the pressure dynamics inside a one of the actuating fluid rails from inducing unwanted pressure dynamics in the other actuating fluid rail.
During normal engine operating conditions, the injectors are actuated at evenly spaced times. When the injector is actuated for injection, the injector control valve opens for an interval and then closes providing the necessary amount of actuating fluid for the injection event in the interval. For an injection event that comprises single shot operation, the injector control valve opens and closes once. For an injection event that includes pilot operation (a small pilot injection followed by a much larger main injection), the valve opens and closes twice or more. When the control valve opens and closes either for a single shot injection event or for a multiple shot injection event, it generates a considerable amount of dynamic disturbance in the actuating fluid in the actuating fluid rail.
First, during the opening period of the control valve, there is relatively large amount of actuating fluid flowing from the actuating fluid rail into the injector for injection actuation. This causes a pressure drop in the actuating fluid rail. This pressure drop is then recovered by the supply actuating fluid flow from the high-pressure pump. Second, the open and close of the injector control valve generates fluid pressure waves along the actuating fluid rail. This pressure wave propagates along the axial direction of the actuating fluid rail with a frequency primarily determined by the length of the actuating fluid rail and the bulk modulus of the actuating fluid.
Since the length of the rail is determined to a large extent by the engine configuration, the frequency varies depending on the engine configuration. For V8 and V6 configurations, the frequency is around 1000–2000 HZ; for 16 configuration, the frequency could be lower due to a longer rail, for example 800–1000 HZ. Because of this pressure wave, there is an unbalanced axial force on the actuating fluid rail since the pressure along the actuating fluid rail is different due to different time delay, or phase lag, at different locations along the actuating fluid rail. This unbalanced force has the same frequency as the pressure wave in the rail. The pressure wave interacts with the actuating fluid rail structure. A fraction of the pressure fluctuation energy converts to the undesirable air-borne acoustic energy. Also, the actuating fluid rail transmits an excitation with the above-mentioned frequency through the bolts connecting the rail to the rest of the engine (for bolt on rails). The same phenomenon occurs in rails formed in the engine structure. In both cases, this excitation then generates an audible noise with the same range of the above noted frequencies.
The audible noise resulting from the acoustic waves is objectionable and must meet NVH (noise, vibration, and harshness) standards. A goal might be that a compression ignition engine be no more noisy than a typical spark ignition engine. Such a level of noise is deemed to be generally acceptable. This is not presently the case, however. In order to meet this goal, a number of sources of noise from the compression ignition engine need to be addressed. As indicated above, one such source is the acoustic emissions generated in the actuating fluid rail. There is then a need in the industry to attenuate the acoustic waves generated in the rail. Further, there is a need for the HEUI type injectors to meet the mandated particulate emissions from the engine by being properly calibrated. Calibration of the injectors depends to a certain extent on the rail dynamics characteristics of the actuating fluid rail.